Opening this up to the public. Please see "Special Design Notes" for details.
tl;dr: This is a set about the world of FFX, not the game of FFX. This set is meant to be played by itself, and doesn't strictly adhere to NWO, so there are a lot of mechanics that are a little more parasitic or complex than they should be. Currently in skeleton filling phase for commons.
Other note: Some of these cards may be a bit unbalanced as of right now. General power level is akin to something like a Masters set.
In the special comments you mention that you don't care about parasitism since the set is meant to be played on its own. Personally, I find it always a little saddening - especially when it's not even that necessary. So I'll be pointing a couple of things related to regardless. I think it's better of a practice to create design patterns that have parasitism to only such extent that's absolutely required.
Flavorfully, I could see Pyreflies being just represented by creature cards in graveyards. So the cards utilizing them could just be delve variants that exile creature cards.
OR if that's definitely not acceptable, at least the cards utilizing Pyreflies could sacrifice enchantments in general - not just Pyreflies specifically.
As for "unnecessary" creature types
Good point. Right now the main concern of mine isn't the parasitism, it's making and play differently - so your suggestion of making the cards that care about saccing them sacrifice enchantments is territory I'd rather not go down. Your suggestion of making the cards exile creature cards from the graveyard is a pretty good one, and I'll probably explore that if Pyreflies don't turn out so well received. That would require a lot of redesigning of the archetypes in general, though. The reason why I prefer Pyreflies at the moment over that is simply due to the "feel" of the set - I'm a little more attached to the "cyclical" themes than I should be, and dying creatures leaving Pyreflies on the battlefield which are then sacrificed to return the creatures has a better feeling than exiling cards in the graveyard to get them back.
Personally, I find that parasitism, while bad for Magic health as a whole, does give ways to make certain sets stand out and feel unique (the deckbuilding & drafting decisions made with Soulshift and Arcane in draft are my favorite parts about Kamigawa gameplay), which is why I'm less wary about using parasitic mechanics on custom sets. The same goes for creature types, which is why I made the Summoner/Aeon type rather than using something like Shaman or Wizard + Incarnation/Avatar/Spirit. Pyrefly is an enchantment type, not a creature type, so Spirit is out the window. I could make them 0/1 enchantment creature tokens, I guess...
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